The present invention relates to a solid state image pickup device which is improved to avoid the reduction in sensitivity caused by the junction capacitance of a photodiode, and to obtain accurate signal output voltage regardless of the change in amount of incident light during an integration period.
In addition to MOS and CCD type devices, there are known amplification type solid state image pickup devices, such as AMI, SIT, CMD, BASIS, FGA, and BCMD. These amplification type solid state image pickup devices do not show problems as far as the size of a pixel is small enough. However, in some applications requiring rather large pixels (such as auto, focusing (AF) control of cameras), these amplification type solid state image pickup devices have problems as described later.
The problems will now be described, while taking into account an AMI (amplified MOS Imager), as an example. FIG. 1 shows a circuit configuration of one pixel of a normal AMI, comprising a photodiode 101, a transistor Q1 for amplification, bias transistors Q2 and Q3, a transistor Q4 for resetting, a bias circuit 102, and a switching transistor 103 driven by output pulses from a shiftregister. The signal output voltage of photoelectric conversion of an AMI configured in such a manner is provided by EQU .DELTA.V.sub.OUT =I.sub.P .multidot.t/Cd (1)
where I.sub.p is a photocurrent, t is an integration period, Cd is a junction capacitance of the photodiode 101. As can be seen from this equation (1), in order to increase the signal output voltage .DELTA.V.sub.OUT under the condition of a constant integration period, it is necessary to increase the photocurrent I.sub.P or decrease the junction capacitance Cd. However, the increase of the photocurrent requires the increase of the area of a pixel. Therefore, this increase of the area of the pixel leads to the increase of the junction capacitance Cd. On the other hand, the reduction of the junction capacitance Cd requires the reduction of the area of the pixel. Subsequently, the reduction of the area of the pixel results in the reduction of the photocurrent. Hence, it is not possible to improve the sensitivity as far as the conventional circuit configuration of the AMI is concerned.
As one of means to solve the above problem, a solid state image pickup device having a configuration shown in FIG. 2 is disclosed in "A New MOS Imager Using Photodiode as Current Source", IEEE Journal of Solid-State-Circuits, Vol. 26, No. 8, Aug., 1991. The configuration of this solid state image pickup device can be obtained from the configuration of the solid state image pickup device shown in FIG. 1 by adding transfer gate transistors Q5 and Q6, and further adding a storage capacitor Ct between the photodiode 101 and the transistor Q1 for amplification. In this configuration of the solid state image pickup device, the transistor Q5 is turned on by data signal during the integration period, so that the transistor Q5 operates in a saturation region, thus the voltage applied to the photodiode 101 is fixed to a voltage lower than the gate voltage of the transistor Q5 by gate-source voltage V.sub.GS. As a result, photo charges generated in the photodiode 101 are stored via the transistor Q5 in the storage capacitor Ct connected to the gate of the transistor Q1 for amplification. Consequently, the affection of the junction capacitance Cd of the photodiode 101 is removed and the signal output voltage .DELTA.V.sub.OUT of the photoelectric conversion is provided by EQU .DELTA.V.sub.OUT =I.sub.P .multidot.t/Ct. (2)
As can be seen from equation (2), the signal output voltage .DELTA.V.sub.OUT can be increased by reducing the capacitance of the storage capacitor Ct. That is, the sensitivity can be determined independently of the junction capacitance of the photodiode.
However, the configuration shown in FIG. 2 of the solid state image pickup device has the following problems. If the amount of incident light is kept constant during the integration period, no problem occurs because the current (photocurrent I.sub.P) flowing in the transistor Q5 is kept constant. However, if the amount of the incident light changes in the integration period, the current flowing in the transistor Q5 changes. As a result, the gate-source voltage V.sub.GS of the transistor Q5 changes, thus a transfer of charges occurs between the junction capacitance Cd of the photodiode and the storage capacitor Ct. This leads to the problem of making it impossible to obtain the accurate photoelectric conversion output.